Osama Edward, director of the Assyrian Network for Human Rights in Syria, and Younan Talia, a senior official in the Assyrian Democratic Organisation, say Sunni tribal sheikhs are trying to establish contact with IS.
Talia said Sunday there has been no response yet.
Islamic State fighters took the captives last week after sweeping through several villages in Syria's northeastern Hassakeh province.
The fate of those kidnapped, almost all of them Assyrian Christians, remains unclear.